a) Field of the Invention
The invention is directed to a microscope with incident light input coupling, wherein the light provided for the incident illumination is directed to the partially reflecting layer of a beam splitter cube and is directed from there through the microscope objective onto the specimen, while the light reflected and/or emitted by the specimen travels back to the partially reflecting layer and passes through the latter into the imaging beam path.
b) Description of the Related Art
Much more than with transmitted light illumination, the quality of images generated with incident light is characterized by controlling the reflections of optical elements in the beam path that is utilized for both illumination and imaging;
these reflections are sometimes extremely troublesome. Therefore, as regards optical design, it must be ensured consistently for each element and each surface in the beam path that the back-reflection of the latter is damped, for instance, by anti-reflection coating of the surface for the relevant spectral region and/or that the position of the ghost image or reflection image of the illumination source (of every surface) is located far away from image planes.
The known steps for eliminating surface reflections on the objectives of the primary tube are directed, above all, to broad-band anti-reflection coating of all lens surfaces of a mounted objective system. In modern design of high-quality objectives for incident light applications, the curvatures of the individual lenses are generally adapted to present-day requirements for low reflection in the intermediate image, aside from the typical requirements for the imaging system for achieving a diffraction-limited transmission of the object plane in the image plane. With most microscope manufacturers, the microscope objectives are corrected to infinity on the image side, i.e., a plane wave surface is generated which first generates a real imaging of the object in the intermediate image plane through the tube lens.
The typical procedure for coupling in the illumination source in incident light applications is neutral, dichroic or polarization-optical input reflection of a parallelized light bundle of the illumination source by means of a plane input-coupling element such as a plane-parallel coated splitter plate or square splitter cube. Accordingly, light is usually coupled into the parallelized infinite beam path between the objective and tube lens. FIG. 1 shows the conventional arrangement for incident illumination in the parallel beam path between the tube lens and objective.
Problems arise with typical incident light input-coupling in the non-parallel, finite beam path of a microscope. When coupling in via an inclined plane plate, aberrations are generated, so that this type of input-reflection of light is not suitable for high-grade optical systems. On the other hand, coupling in via a beam splitter cube between the image plane and tube lens generally generates a relatively strong back-reflection at the lower plane surface of the beam splitter cube. All of the plane surfaces and many optical surfaces with a relatively slight curvature, e.g., the surfaces of the tube lens which usually has a quite long focal length, generate troublesome reflections with incident illumination which reduce the image contrast and imaging quality.
However, particularly with respect to modern microscopy methods, coupling in of incident light between the tube lens and intermediate image plane offers specific advantages, e.g., with respect to the utilization of space in the primary tube of the microscope, so that there is a particular need to solve the problems mentioned above.
An arrangement for low-reflection input-coupling of the illumination source in a parallel confocal incident light microscope is described in DE 19511937 C2. In this case, the neutral, dichroic or polarization-optical input-reflection of the illumination source is carried out above the intermediate image plane and a splitter body which is cut in a rhombic shape is used to prevent troublesome reflections of the plane input-coupling element. In order to reduce the error effect of the rhombus during image formation in the imaging beam path, compensating wedges are required in front of and behind the intermediate image plane. The Nipkow disk, the confocal element in the illumination and imaging beam path, is located in the intermediate image plane and is coated with a broad-band anti-reflection coating, also to prevent troublesome reflections at this element, and arranged in the beam path at a slight inclination.
Another solution for eliminating the troublesome influence of surfaces of plane splitter elements is described in DE 4446134 A1, in which very weak reflections in the fundus oculi must be detected in an interferometric arrangement for measuring the length of the eye. For this purpose, either rhomboidal plane splitter elements or bodies which are slightly skewed but planar are used as splitter elements for coupling in the illumination (semiconductor laser) and for generating a reference beam path of the interferometric measurement principle. These elements are difficult to handle during production as well as for adjustment in the overall arrangement.
In the German Patent DE 19714221 (in contrast to the above-cited DE 19511937 C2), two confocal disks with pinhole arrays are used for preventing strong interfering reflections proceeding from the illumination so that the primary illumination reflections are not even allowed to reach the imaging arm. This is effected at the expense of a complicated adjustment for producing the conjugation of the illumination array to the pinhole array.